or"You can make neither a silk purse out of a sow's ear nor
a race boat out of a fishing boat, but you can come close"by Ken Cook

(NOTE: This article has been edited from the version in B&WB:
a few corrections, clarifications and changes due to HTML format.)

Do I have your attention?
Good! Let's set the egos aside and talk some basic mechanics, hydrodynamics,
physics and math, then go to work on optimizing setup. Top-end speed
is the yardstick by which we usually measure a boat's performance, but
it should not be the objective. "I want to go faster" can be achieved by
simply adding more horsepower and turning a higher pitch prop or more RPM,
but the objective here is to make a particular hull/engine combination
run as efficiently as possible. Each hull design has characteristics that
limit its top end speed capabilities with a given amount of horsepower.
Done properly, setup is
a time consuming process. It can be expensive, and often is, if pursued
to the last MPH.

IN THEORY

Perfecting boat setup requires
the understanding of a few simple "truths" before you can begin. The theory
of optimum boat performance is relatively straightforward, but affected
by many variables:

A prop of "X" pitch will move the hull forward "X" inches if there is zero
slip

A prop MUST slip to work effectively

Drag increases prop slip

Hull drag is a function of wetted surface (area)

Lower-unit drag is a function of gearcase design and engine height

Aerodynamic drag is a factor, but usually a small one until speeds approach
100 MPH

Aerodynamic lift generally decreases hull drag

Some of these variables can
be controlled and some cannot. Careful reworking of the blades can
reduce prop slip that is directly attributable to the prop. Hull
drag can be reduced by lifting more of the hull out of the water (thereby
reducing wetted surface), or by blueprinting the last couple of feet of
the running surface. Lower unit drag may be reduced by raising the
engine, while adding a more hydrodynamic "nosecone" to the gearcase housing
will reduce drag at higher speeds (but may increase drag at slower speeds)
and a condition called "blowout", which occurs at speeds from the high
70s up. Nosecones with low-water pickups allow raising the engine
higher while maintaining sufficient water pressure to keep the powerhead
cool. These variables interact and a change in one may adversely
affect another. For example, you could raise the engine to the point
where the prop can't adequately lift the bow of the boat, so speed slows.
With these basic truths
in mind, you must also understand another important factor. The baseline,
TMS (Theoretical Max Speed), is based on WOT (Wide Open Throttle) RPM,
gear ratio and (effective) prop pitch. Hull size, design and weight,
horsepower and engine setback and height are irrelevant in these
calculations. There are many formulas used to figure a boat's theoretical
top speed, but the simplest mathematical statement is (Tach RPM * Pitch)
/ (1056 * Gear ratio) = TMS. This is straight math and not subject
to debate. If the data plugged into the formula is accurate,
the math doesn't lie. This number is the absolute maximum the boat could
run with that particular pitch prop and engine gear ratio.
To figure out just where
your boat's present setup is compared against the TMS, you need to know
another factor: RWS (Real World Speed), or your boat's speed as measured
by GPS, radar or timing over a measured course. That number, divided by
TMS yields efficiency expressed as a decimal (.90). The difference
between this number and one (1.00) is efficiency loss, usually expressed
as a percentage (10%) and commonly called "slip". I use "efficiency
loss" as it describes total system performance. A 10% efficiency
loss is generally considered good performance for a typical performance
bass boat, while 5-8 % is considered good numbers for a race boat.
For those of you who hate math, the
"Theoretical Performance Chart" is designed to simplify
the math to one division step.

In Real life

Once you know where your
boat is in the scheme of optimum performance according to the numbers,
then it's time to work on set-up to try to improve the performance factor
and reduce that "slip" percentage. The following techniques apply
to any hull, engine and prop combination, whether a 12-foot aluminum jon
boat with 9.5 HP or a 20-plus-foot ultra-high-performance bassboat hull
with a 300 hanging off the transom. But there are no guarantees, nothing
written in stone. Only real-world testing will establish optimum
setup.
To do the job properly,
the following items or accessories are required:

An accurate speed measuring device such as GPS or radar gun

An accurate tach

A jackplate or some other method of readily adjusting engine height

A water pressure gauge

A notebook for recording test information

An assortment of props (one or more additional props on hand can save trips
to the prop shop or dealership)

For larger, higher-horsepower boats,
a foot throttle is a necessity and wheel or floor mounted trim controls
are strongly recommended.
You will also need the following
information:

Environmental conditions play
a significant role. Temperature and humidity can affect engine performance
by several hundred RPM, typically 2-300 lower for a 100 degree day than
for a 50 degree day. A light chop on the water is preferred to "slick"
conditions to minimize "sticking" due to surface tension. An area
with little or no other boating activity and sufficient straight line running
room with good visibility is required for safe testing. Testing should
be done with a typical load of fuel, tackle and people. The objective
is to determine the best the boat can do by controlling as many variables
as possible. Even when optimum setup is achieved, day to day conditions
will usually prevent using full potential.

FINE TUNING SETUP

IMPORTANT: Remember to
wear a PFD and hook up the kill switch.
Monitor the water pressure gauge closely. Each test should consist
of a minimum of two (2) runs at WOT, trimming to achieve maximum speed.
If the boat begins to handle poorly (chine walk, loss of feel in the wheel,
etc.), ease off the throttle and trim down. Repeat the test noting where
performance degrades. Most pad v type hulls will require practice and careful
driving when operating at their performance limits.
When RPM continue to climb
with no increase in speed, the highest trim point for that particular setup
has been exceeded. Once best performance is achieved, note RPM, speed
and amount of trim used. Also note hole shot (B&WB uses 0-30
MPH stopwatch time as a standard), "roostertail" height and handling characteristics.
The roostertail should be no higher than top of cowling for most setups.
This is a sample list of
problems that may be experienced and typical solutions:
1) RPM too high (and not overtrimmed) - a higher pitch prop is indicated
2) RPM too low (and not undertrimmed) - low engine height and/or
prop pitch too high
3) Roostertail too high (and not overtrimmed) - engine too high or
excessive prop slippage (change/rework prop)
There is insufficient room
to list all problems and their solutions in this article. The true
source of a problem can and will vary from one boat to another even if
they are "identical" and must be addressed on an individual basis.
If the engine specifications
have not been exceeded, raise the engine 1/4-1/2" and retest. Note
performance characteristics. Repeat this process as necessary until
water pressure reaches minimum allowable and/or further trim yields RPM
rise without a corresponding speed increase.
You may want to try various
pitch props. Typically, going up or down 1" in pitch can change engine
RPM by 150-200. Going to a smaller pitch prop should make the holeshot
quicker as well as increasing WOT RPM. Likewise, a higher pitch prop
can degrade holeshot and decrease WOT RPM. Sometimes, however, a
higher pitch prop will carry the hull's weight better, resulting in a gain
in both WOTRPM and top speed. Only experimentation will
reveal how your particular hull/engine responds to these changes.
Another factor to consider
is prop blade style. In a nutshell, a three blade prop will typically
run faster than a four blade of the same pitch. Conversely, a four blade
will produce superior holeshot, handle better in rough water and have less
steering torque than the three blade.
Once you have found the
prop style you like, the engine setup where the holeshot is acceptable
and top speed and WOT RPM are maxed out while maintaining minimum safe
engine water pressure, you are at the optimum performance setup for this
hull/engine combination.
Optimizing for maximum speed
does not mean that total performance is optimized. A setup for top
end often sacrifices holeshot, may cause porpoising at low and/or intermediate
speeds and will degrade rough water ride. When set up for rapid holeshot
and good overall top end handling, maximum top speed is usually sacrificed.
Use of an hydraulic jackplate, allowing "on the fly" engine height
adjustment throughout the operating range, can minimize or eliminate these
issues as well as compensate for variations in loading.

SUMMARY

Achieving optimum setup for
maximum performance, although time consuming and often expensive, assures
that you are getting the most out of your investment. Optimum or
near optimum setup pays dividends in higher speed, improved handling and
fuel economy.
Some boaters spend countless
hours fine tuning their boat's setup, playing with different brands and
styles of props, engine setbacks and so forth until they achieve what they
feel is "Nirvana". Others are content to run their rig just as it
came from the dealer. The majority of bassboaters will find a satisfactory
compromise between these two extremes.